Past Papers 2012&2013 Flashcards
Is kWh/yr is a unit of energy?
False
The fundamental units of energy are kg/ms2
False, should be kgm^2/s^2
Individuals hold different viewpoints on the purpose and challenge of creating a sustainable energy supply system. Identify 3 such viewpoints
Viewpoints:
● Human well-being(moral obligation)
● Climate change limitation(save the planet)
● Environment Protection(biodiversity)
● Fossil Fuel Prolongation(sustain economic growth)
● Fossil Fuel Replacement(pollution reduction)
● Security of Supply(political autonomy)
Individuals hold different viewpoints on the purpose and challenge of creating a sustainable energy supply system. Identify how they might be in conflict.
Conflicts
● Energy efficiency measures may worsen indoor air quality
● Decentralised power production reduces global emissions but moves them to
breathing zones
● Increased system complexity -> increased capital/operating/maintenance costs
Elaborate 2 approaches to demand reduction
be employed to create a sustainable energy system in the UK. In each case identify a principal
barrier.
- Use of district heating.
Barrier: Significant capital and continued investment, due to maintenance, required to ensure system robustness
- Unanimous use of biodiesel public transport.
Barrier: Although public transport would put
significantly less strain on the biofuel demand, changing human demands (personal transport) is
a challenge too great.
- Smart grids
Barrier: Requires high levels of communication between lowest levels of demand and higher levels of production, meaning likely high maintenance and general running
costs
- Microgeneration (all forms of microgeneration reduce demand) - CHP.
Barrier: To be considered as sustainable, the input fuel requires to come from a renewable source, such as
biofuels.
Elaborate 2 approaches to energy supply that may
be employed to create a sustainable energy system in the UK. In each case identify a principal
barrier.
- Combined renewable/storage solution. Barrier: Requires a significant investment to achieve the required infrastructure to balance supply, demand, storage and its associated losses, likely at the expense of the user.
- Coal replaced by biofuels. Barrier: Biofuel is not capable of outputting the same amount of power as coal, in MW/Kg, and regrowth has not yet been proven to be sufficient enough to consider biofuel as sustainable.
Give 3 substantial reasons why the renewable energy resource in the UK is likely to be substantially less than the total energy consumption.
● Most renewable sources are intermittent in their performance and it’s very unlikely that their peak supply times will coincide with times of peak demand.
● Demand in the UK is constantly changing, the electricity supply must be able to react immediately to changes in demand- not possible with a renewable supply.
● There are currently no storage capacity in the electricity network- storage would greatly increase the effectiveness of renewable supply.
What impact does climate change have on energy systems and what impact do energy
systems have on climate change?
(The persons answer)
●Climate change will tend to reduce the demand for heating energy while increasing the demand for cooling energy. The overall effect is predicted to be an increase in energy demand.
● The increased consumption of energy is raising temperatures in urban areas – the so-called heat island effect. Systems utilising fossil fuels produce emissions that contribute to (localised) ‘global warming’ and air pollution.
Identify 3 technological challenges facing the continued use of fossil fuels at present rates and give your view on likely effectiveness
1) refine exploration techniques;
2) make less ‘polluting’ (e.g. decarbonise);
3) enhanced extraction (e.g. sequestrate C);
4) new resources (e.g. coal bed methane, oil shale, tar sand);
5) new uses (e.g. methanol production).
Identify the principal emissions from fossil fuel combustion and state how the impact might
be mitigated.
Carbon Dioxide (CO2)
Environmental Problems:
Increasing global surface temperatures
Mitigate Impact: Carbon Capture and storage
Sulphur Dioxide (SO2)
Environmental Problems:
Concentration influences habitat stability for plant and animal communities. Causes acid rain
Mitigate Impact: I think this is the same as the Nitrous Oxides
Nitrous Oxides (NOx)
Environmental Problems:
Respiratory Problems
Mitigate Impact: Careful control of combustion process and treatment of exhaust gases using chemical action or filtration.
Soot - Causes haze
Mitigate Impact: DUNNO
With the aid of a diagram, explain the operation of a combined cycle gas turbine power plant, commenting on its thermal efficiency and how this might be improved.
● CCGT power stations have a secondary steam raising circuit using the heat from
the gas turbine exhaust.
● Conventional gas turbine systems (25-30% efficiency) with massive heat rejection
from exhaust
● CCGT systems (55-60% efficiency)
● Nat Gas used to fuel gas turbine - turbine exhaust temperature is high and
contains a great deal of energy; gases pass through heat exchanger to boil water
and produce steam, which then passes through the turbine that drives a second
generator.
Define Solar Declination
The angle between the equator and a line drawn from the centre of the earth to the centre
of the sun.
Define Solar Azimuth
The azimuth angle is the compass direction from which the sunlight is coming.
Define Solar Altitude
The angular height of the sun in the sky measured from the horizontal.
Define Solar Time
Reckoning of the passage of time based on the sun’s position in the sky. Units - Day
Ts – Tm = ± Ldiff/15 + (Et
/60) + ds
where, Ts = solar time Tm = local time Ldiff = longitude difference Et = equation of time ds = daylight saving time
(The second letter in each term is subscript)
Differentiate between isotropic and anisotropic sky conditions in the context of inclined surface solar irradiance prediction and state why the latter case gives rise to modelling
complexity.
Isotropic: Uniform brightness
Anistropic: Non-uniform brightness
Difficulty in calculating the diffuse component of solar heat gains
With the aid of a sketch, describe a pumped storage hydro scheme
In generating mode, the plant operates as a conventional hydro power plant.
But the generator can function as a motor, and the turbine as a pump.
When a demand for electricity is low, power may be taken from the grid to pump water into the upper reservoir, to act as an energy store for future use when demand for electricity is high
Describe briefly 3(any) types of turbine.
Impulse:
1) Pelton wheel: jet is split symmetrically by blades on the wheel; wheel can have up to 6 jets if mounted on vertical shaft.
2) Turgo: jet passes through the wheel from left to right; it may have a larger diameter than the jet for a Pelton wheel of the same size.
3) Cross Flow: Has a cylindrical runner and allows water to flow through the blades twice (1st pass outside to inside, 2nd pass inside back out). Runner may have several segments on the same axis, to deliver high efficiency for a wide range of flow rates.
4) Runner has simple geometry, may be fabricated from sheet metal.
Reaction:
5) Propeller/Kaplan: an axial-flow machine, normally fitted with guide vanes as shown. If it has adjustable-pitch blades, it is known as a Kaplan turbine. It may also be installed with its shaft
vertical.
Summarise the elements of the UK Renewables Obligation
Licensed electricity suppliers must obtain a specified percentage of their output from renewable sources (closed to new generating capacity from 31 March 2017).
Additional costs incurred may be passed to customers; there are no subsidies.
Arrangements for the trading of Renewables Obligation Certificates (ROCs), or
purchasing exemption from the Obligation.
Electricity Market Reform introduces Contracts for Difference based on agreed ‘strike’ prices as a means to reduce risk for generators.
Summarise the elements of the Renewable Energy Feed-in Tariff
Requires licensed electricity suppliers to make payments to those who install solar photovoltaic, wind, micro-hydro or anaerobic digestion technologies up to a capacity of 5 MW, or micro combined heat and power up to a capacity of 2 kW.
Supplier pays the producer a set rate – the generation tariff – for each unit of electricity generated.
Supplier pays a further rate for each unit exported to the electricity grid – the export
tariff
Explain why energy systems are difficult to model
Because they are complex and subject to changing influences over time
This complexity is because they are dynamic (all part vary at different rates), non-linear(the parameters that define the problem depend on the system conditions that the model is being established to predict), systemic (there are many technical domains each influencing the other – light flow, heat transfer, radiation exchange, control system action, renewable systems etc.) and stochastic (some aspects are random -
e.g. weather and occupant behaviour). To be effective a simulation program must
accommodate all these issues.
Summarise the 2016 Paris Climate Accord and state a principal shortcoming.
To keep global temperatures “well below” 2 C above pre-industrial times and “endeavour to limit” them even more, to 1.5 C
To limit the amount of greenhouse gases emitted by human activity to the same levels that trees, soil and oceans can absorb naturally, beginning at some point between 2050 and 2100.
Rich countries to help poorer nations by providing “climate finance” to adapt to climate change and switch to renewable energy.
National pledges to cut emissions are voluntary; how to enforce pledges has been a stumbling block in talks
Identify 3 principal shortcomings of the Kyoto Protocol.
Three from:
Emissions targets not scientifically based.
Does not address black soot and ozone.
Developing countries excluded.
Share of emissions from international air travel, shipping and importing
manufactured goods not included.
Measures will cause economic burden with no measurable impact.
Explain why a combined cycle gas-fired power plant is more efficient than a conventional coal-fired power plant.
A fossil fuel electricity generating plant uses heat of combustion to boil water generate steam, drive a multi-task turbine and finally a generator. It has an efficiency of about 35%.
A gas fired plant that uses natural gas pumped into gas turbine, where it is mixed with air and burned, converting its chemical energy into heat energy. Alone, this plant has an efficiency of between 25 and 30%
The two plants can be combined by having a secondary steam raising circuit using the heat from the gas turbine exhaust.
Overall, this raises the thermal efficiency to between 55/60%. Primarily due to the relevant temperature differences within each cycle and the Carnot efficiency